In recent years, group III nitride semiconductors have become of interest as a semiconductor material for a light-emitting device that emits light of short wavelength. Such a group III nitride semiconductor is represented by the general formula AlxGayInzN (0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1), and is grown on a substrate of a single crystal sapphire, a variety of oxides, or a group III-V compound, through a metal-organic chemical vapor deposition method (MOCVD method), a molecular-beam epitaxy method (MBE method), or the like.
In a general light-emitting device using a group III nitride semiconductor, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer, each of which is made of a group III nitride semiconductor, are laminated in this order on a single crystal sapphire substrate. Since such a sapphire substrate is an insulator, the device structure generally takes a structure in which a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on the n-type semiconductor layer are present in the same plane. There are two types of such a group III nitride semiconductor light-emitting device: face up type in which a transparent electrode is used as a positive electrode to extract light from the p-type semiconductor side; and flip chip type in which a high reflective film of Ag or the like is used as a positive electrode to extract light from the sapphire substrate side.
External quantum efficiency is used as an index of the output from such a light-emitting device. It can be said that higher external quantum efficiency means a light-emitting device with higher output. The external quantum efficiency is represented as the multiplication of internal quantum efficiency and light extraction efficiency. The internal quantum efficiency is the proportion of energy converted into light in the light-emitting layer amongst energy of electrical current injected into the device. The light extraction efficiency is the proportion of light that can be extracted to the outside of the light-emitting device amongst light generated in the light-emitting layer. Accordingly, in order to improve the external quantum efficiency, the light extraction efficiency needs to be improved.
There are mainly two ways to improve the light extraction efficiency. One is a method for reducing absorption of light into an electrode or the like formed on the light extraction surface. The other one is a method for reducing light confinement within the light-emitting device occurring due to a difference in the refractive index between the light-emitting device and a medium outside of it.
When a transparent electrode is to be provided on a p-type semiconductor so as to improve the light extraction efficiency of a light-emitting device, a metal transparent electrode of Ni/Au or the like has been conventionally used. However, recently, an electrode made of a transparent conductive oxide film of ITO or the like has been used instead. One of the reasons why a metal transparent electrode of Ni/Au or the like has been replaced by a transparent conductive oxide film of ITO or the like, is that the absorption of emitted light can be reduced by using the transparent conductive oxide film.
In addition, as to the method for reducing light confinement within the light-emitting device, a technique for forming a concavo-convex surface on the light extraction surface of the light-emitting device can be enumerated (for example, refer to Patent Document 1).
However, in the light-emitting element in which a concavo-convex surface has been formed on the light extraction surface by means of mechanical or chemical processing, the processing on the light extraction surface causes overloading on the semiconductor layer, leaving damage in the light-emitting layer. In addition, in the light-emitting device in which the semiconductor layer has been grown under such a condition that allows a concavo-convex surface to be formed on the light extraction surface, the crystallinity of the semiconductor layer is deteriorated and thus the light-emitting layer ends up being defective. Therefore, if a concavo-convex surface is formed on the light extraction surface, although the light extraction efficiency is improved, there is a problem in that the internal quantum efficiency is lowered, and thus an increase in the emission intensity can not be achieved.
Here, instead of forming a concavo-convex surface on the light extraction surface, a method for forming a concavo-convex surface on the surface of the sapphire substrate to grow a group III nitride semiconductor layer thereon, has been proposed (for example, refer to Patent Document 2). In this method, the interface between the sapphire substrate and the group III nitride semiconductor layer becomes concavo-convex, and the difference in the refractive index between the sapphire substrate and the group III nitride semiconductor layer causes diffuse reflection of light in the interface, which can reduce the light confinement within the light-emitting device and can improve the light extraction efficiency.
[Patent Document 1] Japanese Patent Publication No. 2836687
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2002-280611